Precision temperature control circuit with improved reliability

ABSTRACT

An oven temperature control circuit includes a four-terminal resistance bridge having a thermistor in one of its arms and a voltage source across one of its diagonals, a transistor differential amplifier with input terminals connected across the other bridge diagonal, an astable transistor multivibrator having its timing controlled from the output terminals of the differential amplifier, a heating element, and a heating current supply transistor having its base-emitter path controlled by an output side of the multivibrator and its emitter-collector path connected to supply current to the heating element from a heating power source. Any departure of the thermistor resistance from a predetermined normal value is detected by the bridge and control currents developed by the differential amplifier are used to vary the duty cycle of the output side of the multivibrator. The heating current supply transistor is switched between the nonconducting and saturated states and its duty cycle varied along with that of the output side of the multivibrator, thereby supplying power to the heating element without substantial dissipation in the heating current supply transistor.

United States Patent Thelen I PRECISION TEMPERATURE CONTROLCIRCUIT WITHIMPROVED RELIABILITY [72] Inventor: William Thelen, Glen Ellyn, Ill.[73] Assignee: Bell Telephone Laboratories, Ineor porated, Murray Hill,Berkeley Heights, NJ.

[22] Filed: Sept. 1, 1971 [211 App]. No.: 176,827

[52] US. Cl. ..219/501, 219/497, 219/499 [51] Int. Cl. ..H05b 1/02 [58]Field of Search ..219/497, 499, 501, 505

[56] References Cited UNITED STATES PATENTS 3,586,829 6/1971 Farmer eta1. ..219/497 2,994,759 8/1961 Lipman ..219/501 PriniaryExaminer-Bernard A. Gilheany Assistant Examiner-F. E. Bell Attorney-R.J. Guenther et al.

[ Nova 14, 1972 [57] ABSTRACT An oven temperature control circuitincludes a fourterminal resistance bridge having a thermistor in one ofits arms and a voltage source acres one of its diagonals, a transistordiiferential amplifier with input terminals connected across the otherbridge diagonal, an astable transistor multivibrator having its timingcontrolled from the output terminals of the differential amplifier, aheating element, and a heating current supply transistor having itsbase-emitter path controlled by an output side of the multivibrator andits emitter-collector path connected to supply current to the heatingelement from a heating power source, Any departure of the thermistorresistance from a predetermined normal value is detected by the bridgeand control currents developed by the differential amplifier are used tovary the duty cycle of the output side of the multivibrator. The heatingcurrent supply transistor is switched between the nonconducting andsaturated states and its duty cycle varied along with that of the outputside of the multivibrator, thereby supplying power to the heatingelement without substantial dissipation in the heating current supplytransistor.

4 Claims, 3 Drawing Figures PATENTEDmw 14 m2 F/az POWER DISSIPATION maxHEATER CURRENT PRECISION TEMPERATURE CONTROL CIRCUIT WITH IMPROVEDRELIABILITY BACKGROUND OF THE INVENTION This invention relates generallyto oven temperature control circuits and more particularly to electronictemperature control circuits with exceptionally high requirements forreliability and precision.

In large electronic communication systems such as high capacitytelephone switching and multiplex transmission systems, it is oftenimportant that some components with operating characteristicsparticularly critical important timing, frequency, phase, or signalamplitude relationships be maintained at as nearly a constanttemperature as possible. Under such circumstances, the criticalcomponents are typically housed in an oven and control circuitry isprovided to keep the oven temperature substantially unvarying. In thecontrol circuitry, a premium is placed not only upon accuracy, which isneeded to minimize oven temperature variations, but also reliability,which is important in order to avoid any frequent necessity for takingthe entire communication system out of service or permitting it tooperate at a reduced performance level while temperature control circuitcomponents are being replaced. 7

Although many oven temperature control circuits found in the prior artprovide a high degree of precision in temperature control, they tend notto have the desired extremely high degree of reliability. Controlcircuits using bimetallic or mercury sensing elements rely upon theopening and closing of mechanical contacts for their operation and suchcontacts have a tendency to fail after prolonged use. Electroniccircuits using thermistor bridge detectors to sense temperaturevariations and amplifying devices to control the flow of heatingcurrents avoid the particular problems associated with mechanicalcontacts but still tend to have reliability problems caused by failureof the electronic amplifying devices supplying current to the ovenheaters. Vacuum tubes have limited useful lifetimes because of suchfactors as heater burnout and cathode deterioration, and transistorstend to fail because of excessive power dissipation within their ownemitter-collector paths.

A principal object of the invention is to improve the reliability of thetype of electronic temperature control circuit using one or moretransistors to control the flow of heating current without sacrifice ofprecision.

Another and more particular object is to reduce the power dissipation inthe heating current supply transistors in an electronic temperaturecontrol circuit without sacrifice of precision.

SUMMARY OF THE INVENTION In a temperature control circuit in accordancewith the invention, departures of temperature from a predeterminednormal value are detected, the heating current supply transistor ortransistors are switched between substantially nonconducting andsubstantially saturated states, and the duty cycle of the heatingcurrent supply transistor or transistors is varied under the control ofany detected departures in order to provide the required heatingcorrections. Power dissipation in the emitter-collector paths of theheating current supply transistors is drastically reduced andreliability increased because practically no current flows while thetransistors are in their nonconducting state and the amount of seriesresistance is quite low while they are in their saturated state. Thepresent invention thus reduces power dissipation in the heating currentsupply transistors of an electronic temperature control circuit in amanner somewhat reminiscent of that employed by a voltage regulator ofthe so-called switching type to improve regulator efficiency, with theimportant difference that the temperature control circuit, incompensating for such changes as shifts in ambient temperature,functions to provide an electrical output which is essentially varyingwhile a voltage regulator functions to maintain an electrical outputwhich is substantially constant.

An important embodiment of the invention takes the form of a temperaturecontrol circuit which includes a four-terminal resistance bridge havinga thermistor in one of its arms and a voltage source across one of itsdiagonals, a differential amplifier having input terminals connected toopposite ends of the other bridge diagonal, an astable or free-runningmultivibrator having its timing controlled from the differentialamplifier output terminals, and at least one transistor having itsbase-emitter path coupled to an output side of the multivibrator and itsemitter-collector path connected to supply current to a heating elementfrom a heating power source. In this embodiment, the active bridgedetects any departure of the thermistor resistance from a predeterminednormal value and translates it into a potential difference across theoutput diagonal. The differential amplifier supplies current to and fromthe multivibrator which varies the duty cycle of the output side of themultivibrator and the multivibrator output switches theemitter-collector path of the heating current supply transistor betweensubstantially nonconducting and substantially saturated states, therebysupplying power to the heating element and controlling the heatgenerated without substantial dissipation in the heating current supplytransistor itself and with no sacrifice of precision. Both the bridgethermistor and the heating element are, in practice, typically placedwithin the oven the temperature control circuit is designed tocontroL-Significant additional benefits realized include a very lownoise sensitivity at extreme duty cycles because of the substantiallylinear multivibrator timing current discharge afforded by thedifferential amplifier.

A more complete understanding of the invention and its features may beobtained from a study of the following detailed description of aspecific embodiment and the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of aspecific embodiment of the invention using an active bridge, adifferential amplifier, and an astable multivibrator to switch theheating current supply transistors.

FIG. 2 is a curve showing the manner in which power dissipation in theheating current supply transistor varies with the amount of heatingcurrent supplied.

FIG. 3 is a schematic diagram of an alternative temperature variationdetecting bridge suitable for use in the embodiment of the inventionshown in FIG. I to provide still better temperature control precision.

DETAILED DESCRIPTION In the embodiment of the invention illustrated inFIG. 1, a thermistor 11 is connected in a four-terminal bridgeconfiguration with four resistors 12, 13, 14, and 15. As shown,thermistor 11 is connected in series with resistor 12 in one arm of thebridge, each of the remaining resistors forms one of the remaining arms,and a d-c voltage source 16 is connected across one of the bridgediagonals. In detail, thermistor 11 and resistors 12 and 13 form oneseries path and resistors 14 and 15 form another between the positiveside of source 16 and ground. Resistors 13, 14, and 15 all havesubstantially the same resistance, for example, and that resistance issubstantially the same as the series resistance of resistor 12 andthermistor 11 at an intermediate point in the operating temperaturerange of the latter. The resistance of resistor 12 may be adjusted, ifdesired, to change the temperature setting of the control circuit. Anyof resistors 13, 14, and 15 may, of course, be used to provide the sameadjustment capability and resistor 12 eliminated. Thermistor 11 wouldthen have substantially the same resistance as resistors 13, 14, and 15.

A transistor differential amplifier has its input terminals connected toopposite ends of the other diagonal of the four-terminal bridge shown inFIG. 1. Transistors 17 and 18 are a matched pair of n-p-n transistorsand have their emitter electrodes connected through a common biasingresistor 13 to ground. The base electrode of transistor 17 is connectedto the bridge terminal formed by the junction between resistors 12 and13, and that of transistor 18 is connected to the bridge terminal formedby the junction between resistors 14 and 15. The collector electrodes oftransistors 17 and 18 are the output terminals of the differentialamplifier and present relatively large transistor collector impedances.

The output terminals of the differential amplifier in FIG. 1 areconnected directly to timing control terminals of an astable orfree-running transistor multivibrator. This multivibrator includes apair of p-n-p transistors 22 and 23.

As illustrated, the collector electrode of differential amplifiertransistor 17 is connected directly to the base electrode ofmultivibrator transistor 22 and the collector electrode of differentialamplifier transistor 18 is connected directlyto the base electrode ofmultivibrator transistor 23. Current limiting resistors 24 and 25 areconnected from the collector electrodes of respective transistors 22 and23 to ground, and the emitter electrodes of transistors 22 and 23 arereturned through a common diode to the positive side of voltage source16. As shown, diode 26 is poled for easy current flow in the directionfrom source 16 toward transistors 22 and 23. Timing capacitors 27 and 28complete the multivibrator, with capacitor 27 providing crosscouplingbetween the collector electrode of transistor 22 and the base electrodeof transistor 23 and capacitor 28 providing similar cross-couplingbetween the collector electrode of transistor 23 and the base electrodeof transistor 22. As illustrated, transistor 23 forms the output side ofthe multivibrator.

The remainder of the oven temperature control circuit in FIG. 1constitutes the actual heating current supply circuit. A resistor 31couples the collector electrode of multivibrator output transistor 23 tothe base electrode of an n-p-n amplifying transistor 32. A load resistor33 is connected from the collector electrode of transistor 32 to thepositive side of a heating power source 34, and a current limitingresistor 35 is returned from the emitter electrode of transistor 32 toground. Finally, the emitter electrode of transistor 32 is connecteddirectly to the base electrode of the actual n-p-n heating currentsupply transistor 36, the oven heating element 37 is connected from thecollector electrode of transistor 36 to the positive side of heatingpower source 34, and the emitter electrode of transistor 36 is returnedthrough a current limiting resistor 38 to ground.

In the illustrated embodiment of the invention, input bridge thermistor11 and heating element 37 are typically enclosed within the oven theyare intended to control. If a change in oven temperature causes theresistance of thermistor 11 to change, the bridge becomes unbalanced anda potential difference appears across the diagonal between the junctionof resistors 12 and 13 and the junction of resistors 14 and 15. Thetransistor differential amplifier then acts as a current steeringcircuit, increasing the current flowing from the multivibrator into thecollector electrode of one of transistors 17 and 18 and decreasing thatflowing into the other. The astable or free-running multivibrator inFIG. 1 is conventional except that its timing is controlled from thethermistor bridge by way of the transistor differential amplifier.Timing capacitors 27 and 28 discharge into the relatively high collectorimpedances of differential amplifier transistors 17 and 18.

In the operation of the illustrated embodiment of the invention, theduty cycle, i.e., the percentage of the operating cycle during which thetransistor is conducting, of each side of the multivibrator is changedwhen temperature variations unbalance the bridge. When the resistance ofthermistor 11, which typically has a negative temperature coefficient ofresistance, drops because of a rise in temperature, the collectorcurrent drawn by transistor 17 decreases and that drawn by transistor 18increases. Timing capacitor 27 in the multivibrator thus discharges morequickly and timing capacitor 28 discharges more slowly, causing the dutycycle of transistor 22 to increase and that of transistor 23 todecrease. When the resistance of thermistor ll rises because of a dropin temperature, the action is just the opposite. The collector currentdrawn by transistor 17 then increases and that drawn by transistor 18decreases. Timing capacitor 27 in the multivibrator discharges moreslowly and the timing capacitor 28 discharges more rapidly, causing theduty cycle of transistor 22 to decrease and that of transistor 23 toincrease.

As illustrated in FIG. 1, the output from the multivibrator is takenfrom the collector electrode of transistor 23. When multivibrator outputtransistor 23 conducts, the output voltage rises to a value approachingthe voltage of positive voltage source 16 and, when transistor 23 isnonconducting, that voltage drops to substantially zero. The outputvoltage at the collector electrode of transistor 23 is thus asubstantially square wave the duty cycle of which varies inversely withthe temperature of thermistor 11. The output waveform is applied throughcoupling resistor 31 to the base-emitter path of transistor 32.Transistor 32, which is an emitter follower, acts as a non-invertingpower amplifier and supplies the same waveform to the baseemitter pathof heating current supply transistor 36. When the output voltage fromthe multivibrator is high, transistor 36 receives a strong forward biasacross its emitter-base path which causes transistor 36 to saturate andits emitter-collector path to switch into a very low impedance state.When the output voltage from the multivibrator is low, the forward biason transistor 36 is eliminated or sharply reduced and itsemitter-collector path switches back into a high 'impedancestate. Theduty cycle of heating current supply transistor 36 is thus varied in thesame manner as that of multivibrator output transistor 23. l

The average heating current flowing in heating element 37 in theembodiment of the invention shown in FIG. 1 varies directly with theduty cycle of heating current supply transistor 36 and provides a nearlylinear relationship between oven temperature and heater power. Thermaldelay in heating element 37 provides an averaging effect whicheffectively smooths the on-off pattern of the heatingcurrent. Increasesin the duty cycle increase oven temperature and decreases in the dutycycle have the opposite effect.

The manner in which power dissipation in the internal emitter-collectorpath of heating current supply transistor 36 in FIG. 1 varies with theamount of heater current flowing is illustrated in FIG. 2. As shown,dissipation is zero when no heater current is flowing. Although theinternal emitter-collector path of the transistor then has relativelyhigh resistance, no power is dissipated because no current flows. Ascurrent begins to flow through the emitter-collector path of transistor36, power is dissipated within the transistor. The amount of powerdissipation increases as current increases, reaches a maximum, and thendecreases as the transistor approaches saturation. When the transistorreaches saturation, the internal resistance of its emitter-collectorpath is very low and, even though the current is maximum, the powerdissipation again approaches zero. The present invention takes advantageof this characteristic feature of transistors by switching theemitter-collector path of transistor 36 back and forth between the twosubstantially zero power dissipation points on the curve. As a result,power dissipation in heating current supply transistor 36 remains low atall times and the desired improvement in circuit reliability isachieved.

Another important advantage afforded by the embodiment of the inventionillustrated in FIG. 1 is a very low noise sensitivity at extreme dutycycles. Multivibrator timing capacitors 27 and 28 discharge directlyinto the collector electrodes of differential amplifier transistors 18and 17, respectively. Because the collector impedances of transistors 17and 18 are large, timing capacitors 27 and 28 discharge on curves whichare substantially linear. At any point in time, whichever one of timingcapacitors 27 and 28 is discharging retains more charge than it would ifit were discharging along a curve which approached asymptotically thelevel at which the multivibrator triggers and reverses state. For thisreason, small power supply disturbances, which may appear as noise, areless likely to cause premature triggering and reversal of state withinthe multivibrator and the duty cycles of both sides are less likely tobe disturbed.

Because heating element 37 in FIG. 1 is nonnally located within the ovenbeing controlled in close physical proximity to critical communicationsystem components, there is always a possibility that sharp switchingtransients in heating current supply transistor 36 may be coupled,through inductive or capacitive cross-talk, into such components asnoise. If that occurs, the noise may be eliminated with but little lossin heating current supply transistor reliability by providing a slightslope to the leading and trailing edges of each heating current pulse.This slope or ramp may be provided by connecting a small capacitor toground from the'base electrode of transistor 32 and by connecting aresistor in parallel with the capacitor. There is a slight increase inpower dissipation in the emitter-collector path of heating currentsupply transistor 36 as a result but, as long as the slope is confinedto a small fraction of each heating current pulse, any adverse effectupon circuit reliability is minor.

Finally, it is advantageous that power transistors 32 and 36 anddifferential amplifier transistors 17 and 18 are all of the n-p-nvariety. In the present state of transistor fabrication art, n-p-n powertransistors are somewhat easier to manufacture than p-n-p powertransistors, and n-p-n transistors are easier than p-n-p transistors tomanufacture in matched pairs. It should be understood, though, that theinvention is in no way limited to any particular transistor conductivitytype and that the illustrated embodiment is entirely operative withtransistors of opposite conductivity type substituted for those shown aslong as the polarities of all power sources and diodes are alsoreversed.

An alternative temperature variation detecting bridge circuit which maybe used in the embodiment of the invention illustrated in FIG. 1 toincrease precision still further with no loss in reliability is shown inFIG. 3. There, the active bridge is the same as that shown in FIG. 1except for the addition of another stage of differential amplificationand connects to the remainder of the FIG. 1 circuitry along the lineA-A.

In FIG. 3, a second matched pair of n-p-n transistors 41 and 42 form asecond differential amplifier stage which drives transistors 17 and 18.As shown, the base electrode of transistor 41 is connected to thejunction between bridge resistors 12 and 13 and the base electrode oftransistor 42 is connected to that between bridge resistors 14 and 15. Apair of resistors 43 and 44 are connected from the respective collectorelectrodes of transistors 41 and 42 to a common point which is connectedthrough a resistor 45 to the positive side of voltage source 16 andthrough a zener diode 46 to ground. Zener diode 46 is connected, asshown, with its forward direction of conduction from ground toward thecommon point between resistors 43 and 44. Resistor 45 and diode 46 shiftthe voltage level of the collector electrodes of transistors 41 and 42to enable those transistors to function effectively with differentialamplifier transistors 17 and 18. The emitter electrodes of transistors41 and 42 are connected together and returned to ground through a commonbiasing resistor 47.

Because the second stage of differential gain shown in FIG. 3 provides aphase inversion, the connections from difierential amplifier transistors17 and 18 are reversed from those shown in FIG. 1. In FIG. 3, thecollector electrode of transistor 17 is connected to the base electrodeof multivibrator transistor 23, while that of transistor 18 is connectedto that of multivibrator transistor 22.

What is claimed is:

l. A temperature control circuit which comprises means to detectdepartures of tem erature from a predetermined value, a heating element,a transistor having its emitter-collector path connected to supplycurrent from a first d-c power source to said heating element, anastable multivibrator having an output terminal and at least one timingcontrol terminal, means coupling said detection means to saidmultivibrator timing control terminal to vary the duty cycle at saidmultivibrator output terminal, and means coupling said multivibratoroutput terminal to the base-emitter path of said transistor to switchthe emitter-collector path of said transistor between substantiallynonconducting and substantially saturated states, thereby supplyingpower from said first power source to said heating element andcontrolling the amount of heat generated thereby without substantialdissipation in the emittercollector path of said transistor.

2. A temperature control circuit in accordance with claim 1 in whichsaid detection means comprises a four-terminal resistance bridge havinga temperaturesensitive resistance in one of its arms and a second d-cpower source connected across one of its diagonals, said bridgedetecting any departure of the resistance of said temperature-sensitiveresistance from a predetermined value and translating it into apotential difference across the other of its diagonals.

3. A temperature control circuit in accordance with claim 2 in whichsaid temperature-sensitive resistance is a thermistor. I

4. A temperature control circuit which comprises a four-terminalresistance bridge having a thermistor in one of its arms and a first d-cpower source connected across one of its diagonals, said bridgedetecting any departure of the resistance of said thermistor from apredetermined value and translating it into a potential differenceacross the other of said diagonals, a differential amplifier having apair of output terminals and having a pair of input terminals connectedto opposite ends of said other of said bridge diagonals, an astablemultivibrator having an output terminal and a pair of timing controlterminals, said differential amplifier output terminals being connectedto respective ones of said multivibrator timing control terminals tovary the duty cycle at said multivibrator output terminal under thecontrol of said differential amplifier, a heating element, a transistorhaving its emitter-collector path connected to supply current form asecond d-c power source to said heating element, and means coupling saidmultivibrator output terminal to the base-emitter path of saidtransistor to switch the emitter-collector path of said transistorbetween substantially nonconducting and substantially saturated states,thereby supplying power from said second source to said heating elementand controlling the amount of heat generated thereby without substantialdissipation in the emittercollector path of said transistor.

1. A temperature control circuit which comprises means to detectdepartures of temperature from a predetermined value, a heating element,a transistor having its emitter-collector path connected to supplycurrent from a first d-c power source to said heating element, anastable multivibrator having an output terminal and at least one timingcontrol terminal, means coupling said detection means to saidmultivibrator timing control terminal to vary the duty cycle at saidmultivibrator output terminal, and means coupling said multivibratoroutput terminal to the baseemitter path of said transistor to switch theemitter-collector path of said transistor between substantiallynonconducting and substantially saturated states, thereby supplyingpower from said first power source to said heating element andcontrolling the amount of heat generated thereby without substantialdissipation in the emitter-collector path of said transistor.
 2. Atemperature control circuit in accordance with claim 1 in which saiddetection means comprises a four-terminal resistance bridge having atemperature-sensitive resistance in one of its arms and a second d-cpower source connected across one of its diagonals, said bridgedetecting any departure of the resistance of said temperature-sensitiveresistance from a predetermined value and translating it into apotential difference across the other of its diagonals.
 3. A temperaturecontrol circuit in accordance with claim 2 in which saidtemperature-sensitive resistance is a thermistor.
 4. A temperaturecontrol circuit which comprises a four-terminal resistance bridge havinga thermistor in one of its arms and a first d-c power source connectedacross one of its diagonals, said bridge detecting any departure of theresistance of said thermistor from a prEdetermined value and translatingit into a potential difference across the other of said diagonals, adifferential amplifier having a pair of output terminals and having apair of input terminals connected to opposite ends of said other of saidbridge diagonals, an astable multivibrator having an output terminal anda pair of timing control terminals, said differential amplifier outputterminals being connected to respective ones of said multivibratortiming control terminals to vary the duty cycle at said multivibratoroutput terminal under the control of said differential amplifier, aheating element, a transistor having its emitter-collector pathconnected to supply current form a second d-c power source to saidheating element, and means coupling said multivibrator output terminalto the base-emitter path of said transistor to switch theemitter-collector path of said transistor between substantiallynonconducting and substantially saturated states, thereby supplyingpower from said second source to said heating element and controllingthe amount of heat generated thereby without substantial dissipation inthe emitter-collector path of said transistor.